Semiconductor lasers such as vertical-cavity surface-emitting lasers (VCSELs) and edge-emitting lasers (EELs) that emit at visible wavelengths in the range of about 400 to 700 nm are of particular interest for applications such as optical scanning, image display, laser printing and xerography, optical data storage and readout, and plastic-fiber-based optical communications. These lasers may also be integrated with other electronic and photonic devices such as transistors and photodetectors for applications such as optical interconnects and the like. Of particular interest are VCSELs that emit laser radiation in a direction perpendicular to the plane of a semiconductor p-n junction formed within an optical cavity thereof since these devices emit a low-divergence output beam.
Olbright et al U.S. Pat. No. 5,258,990 discloses a visible VCSEL having undoped InAlGaP spacer layers disposed between the active region and distributed Bragg reflector (DBR) mirrors. The use of undoped InAlGaP spacer layers is disadvantageous due to the relatively high electrical resistance resulting from not doping the InAlGaP spacer layers. Furthermore the use of InAlGaP as a material for the spacer layers is disadvantageous due to the relatively high thermal resistance of InAlGaP.
Schneider et al U.S. Pat. No. 5,351,256 which is incorporated herein by reference discloses a visible VCSEL that is an improvement over the patent of Olbright et al and provides the first demonstration of an electrically-injected visible VCSEL. The advance of Schneider et al was to use InAlP optical-phase-matching spacer layers on either side of the active region to improve carrier confinement within the active region, and to reduce electron leakage therefrom. Although this improvement allowed electrically-injected pulsed lasing operation at a duty cycle of up to 40%, further improvements are needed for lasing in a continuous-wave (cw) mode.
A disadvantage of the visible VCSEL design of Schneider et al is the preference for relatively thick InAlP spacer layers (from 1.lambda., to 3.lambda. or more). Such thick InAlP spacer layers increase an optical loss within an optical cavity of the device due to increased free carrier absorption and impurity scattering therein.
Another disadvantage of the Schneider et al VCSEL design is the use of a relatively thick active region (2.lambda.) which reduces the carrier injection efficiency, increases the optical loss of the optical cavity, and produces excessive device heating within the optical cavity due to the poor thermal conductivity of InAlGaP. This excessive device heating, which is indicated by lasing in an n=2 transition, limited operation of the VCSEL to a pulsed mode; whereas cw operation is preferred for many visible VCSEL applications.
A third disadvantage of the Schneider et al device is the use of magnesium (Mg) as a dopant for the p-type InAlP spacer layer. Mg may diffuse and exhibit erratic transport behavior during growth by metal-organic chemical vapor deposition (MOCVD) and may accumulate near the heterointerface between the p-type InAIP spacer layer and adjacent DBR mirror.
Yet another disadvantage of the Schneider et al patent is that growth of the InAlP spacer layers immediately adjacent to the AlAs/AlGaAs DBR mirrors is made difficult by requiring an abrupt and complete switch-on of a P source in the MOCVD growth during epitaxial growth at a high temperature of about 750.degree.-775.degree. C. And for growth of an InAlGaP active region another abrupt and complete switch-on of an As source must be performed. Each switch-on must be done very quickly, and is disadvantageous from a manufacturing standpoint due to the possibility of producing uncertain heterointerface characteristics between the DBR mirrors and the spacer layers.
An advantage of the visible semiconductor laser according to the present invention is that carbon may be used exclusively as a p-type dopant, thereby improving the dopant precision and stability against thermal diffusion during epitaxial growth and subsequent device processing.
Another advantage according to some embodiments of the visible semiconductor laser of the present invention is that one or more relatively thin (.ltoreq.1.lambda.) transition layers may be provided to reduce band offsets between an InAlGaP active region and surrounding AlGaAs layers.
A further advantage of embodiments of the present invention having one or more transition layers is that the transition layers may be formed of a substantially indium-free semiconductor alloy such as AlAsP, AlGaAsP, or the like so that the transition layers may be doped with carbon for forming a semiconductor laser having carbon as the sole p-type dopant.
Yet another advantage of embodiments of the present invention having one or more transition layers is that the transition layers may have a nearly-continuous composition grading without the need for any abrupt switch-on of As and P sources, thereby improving epitaxial growth and minimizing interface uncertainties.
Still another advantage of embodiments of the present invention having one or more transition layers is that the transition layers may provide for an increased thermal conductivity and reduced heating within the visible semiconductor laser by reducing a thickness of epitaxial layers in the device and/or by allowing the use of Al.sub.x Gal.sub.x As materials surrounding the InAlGaP active region with a thermal conductivity higher than InAlGaP or InAlP.
These and other advantages of the visible semiconductor lasers of the present invention will become evident to those skilled in the art.